1. Field of the Invention
This invention relates to a cam mechanism for driving reciprocating machines and to a constant volume pulsation-free reciprocating pump utilizing said cam mechanism.
2. Description of the Prior Art
Reciprocating pumps perform pumping action consisting in the suction and delivery of liquid by the reciprocating motion of a plunger, piston or the like (hereinafter referred to collectively as a plunger) in a cylinder. Particularly, the plunger pump, as compared with others, withstands very high pressure and is superior in volumetric efficiency, so that it has been widely used in various fields. However, the delivery action of reciprocating pumps is intermittent and the delivery rate varies to a great extent, causing the so-called pulsation. If the delivery rate is not constant at any moment, as described above, various inconveniences arise. Particularly, it cannot be put to use in process plants or other facilities which require a constant volume of liquid to be fed at all times.
Generally, the reciprocating pump comprises a liquid pressing chamger, a cylinder, and a plunger axially slidably installed in the cylinder to allow its one end to come in and out of the pressing chamber. The pressing chamber communicates on one hand with a liquid suction port via a suction valve allowing the passage of only liquid flowing into the pressing chamber and on the other hand with a delivery port via a delivery valve allowing the passage of only liquid flowing out of the pressing chamber. Heretofore, ball valves and poppet valves have been used as such suction valves and delivery valves. This type of valve performs an opening and closing action, that is, it is opened by liquid pressure and closed by gravitation; thus, it is a so-called check valve allowing the passage of liquid only in one direction. There is another type in which a spring is installed in the valve mechanism to urge the valve body in one direction. In such conventional valves, however, the valve body, in closing, has to sit on the valve seat under gravitation, during which it sinks while abutting against the corners of the valve seat, making the valve closing timing inaccurate. If the closing of the valve is delayed, the liquid will flow back, leading to a variation in the delivery rate. Further, a time lag is also caused to the valve opening timing owing to surge during the opening of the valve. Even in the case of using a spring, it is necessary to use a strong spring in order to secure a stabilized valve closing timing, but because of the structural limitation requiring that such spring should be installed in the valve mechanism, it could not be expected to employ a sufficiently strong spring. Even if it can be employed, the strength of such spring, in turn, acts as a suction resistance to the suction valve or causes cavitation. For the delivery valve, it is only when the relation of pressures on both sides of the valve body during delivery stroke is that: EQU (liquid pressure on delivery side)+(spring pressure&lt;(liquid pressure on pressing chamber side)
that the valve is opened, with the result that the spring pressure shares the cause of the pulsation of the delivery rate.
Further, materials for the valve body suitable for the handling of corrosive liquid include stainless steel, ceramics, titanium, and Hastelloy. Stainless steel cannot be said to be universally corrosion resistance, while ceramics, titanium and Hastelloy are too light in weight to rely on gravitation for the closing of the valve. Even if a spring is to be built in, it would be very difficult to obtain a spring designed to meet both requirements for corrosion resistance and spring constant. In the case of a valve of the conventional type using gravitation or an internal spring of limited spring pressure when handling viscous liquid, the valve opening and closing action could be slow or, at times, the valve could not move at all owing to the viscosity of liquid.
In the case of driving by using a cam mechanism, a spring is generally used for the returning of the plunger. However, as a characteristic of a spring, there is a difference in pressure between the time the spring is expanded and the time it is contracted; thus, to match with all operating conditions it is inevitable to use a considerably strong pressure, leading to the consumption of unnecessary energy and, moreover, durability has to be taken into account. It would be also possible to use a grooved cam, but in the case of a constant volume pump requiring a high degree of accuracy, the gap between the cam groove and the cam follower becomes a problem, causing difficulty to the production and assembly of the pump.